The inspection of brittle articles may be carried out in a number of different ways, of which simple visual inspection is perhaps the most common. Apart from visual inspection, there are on existence, or there have been proposed, automatic or semi-automatic techniques for inspecting glass bottles and jars so as to detect flaws such as cracks. Such techniques are generally optical in nature and rely on light-scattering phenomena.
In connection with fine, hand-made circular glass and ceramic ware, particularly articles such as vases, bowls, drinking glasses, cups, plates and the like, it is common practice to test the finished article (after firing in the case of ceramics) for soundness by applying a light, sharp impact with the fingers, so as to cause the article to resonate with an audible note. The skilled operative can tell from the quality of this note whether there is any fault in the article. The principle of using resonant vibration in the audio range, produced by impact, is applied to a proposal for repetitive industrial inspection of one specific product, viz. plastics-coated glass bottles, contained in the United Kingdom patent (GB-A-1416082) granted to Dart Industries Inc. In the method described in GB-A-1416082, a positive impact force is applied to the bottle, and the intensity of the vibrations within the audio range 50 Hz-10,000 Hz, emitted by the bottle after a predetermined lapse of time, is then detected, by means of a transducer in contact with the bottle, and measured. In a good bottle, i.e. one having no cracks, the intensity after the pre-determined time lapse is known for any particular design and size of bottle and a given impact force, so that this value of the intensity may be used as a datum. If the bottle is cracked, the resonance will be damped by the crack; therefore if the intensity after the predetermined time lapse is significantly less than the datum value, the bottle is identified as being cracked.
It should be noted that the method of GB-A-1416082 must depend for its reliability on the comparison, between the sonic intensity produced in a given bottle and that used as a datum value, being itself a reliable criterion. This in turn requires that each bottle tested must, in its undamaged state, display no more than a small variation from the characteristics adopted for the "datum" bottle. In this connection, there is commonly found, in the manufacture of glass articles by mass production methods, a substantial variation in various dimensions, such that the intensity of the audible resonance, at the point on the article chosen for contact with the transducer, may show some considerable variation even in the absence of a crack. Close control of dimensional variations therefore may need to be applied.
In addition, the need to apply an impact force to a brittle article, whilst not dangerous in the case of a fine handmade article whose degree of symmetry is likely to be high (so that the force can be very small indeed and still produce an easily-detectable note), may often be dangerous in the case of articles made by repetitive industrial processes. This is because the magnitude of the impact force must be sufficient to produce resonance on all of a succession of articles, regardless of any dimensional variations, and will therefore need to be more substantial than in the case quoted above.
In addition, a method such as that proposed in GB-A-1416082 is only suitable in respect of articles whose shape is such that they will resonate in the audio range of frequencies when struck. This in general restricts the application of such a method to articles which are generally circular. Even then, it is not in general applicable to any article which is in a condition such that any resonance will be damped to a degree at which it cannot be effectively measured. This applies for example to many containers which have been filled with a product (particularly a solid or heavy viscous liquid product).
Once a container has been filled, it is desirable for a number of reasons to apply a closure immediately; and a filled container, for present purposes, implies a container that has an appropriate closure secured on to it; whilst a closed container implies a filled container.
Closed containers of glass or other brittle material are not in practice usually subjected to inspection for damage, other than visual inspection. In fact, since glass, if handled with reasonable care, is quite strong, a closed glass bottle or jar is unlikely to suffer damage, other than outright breakage, between the completion of the closing process and initial removal of the closure by the eventual consumer. This is probably because the most vulnerable part of the container is its neck or "finish" to which the closure is secured, since the closure extends around and over the finish and affords it some considerable measure of mechanical protection.
In this connection it should be noted that in this specification reference to closures means more particularly closures, of plastics or metal, of the kind generally referred to as caps, i.e. those which do have an external skirt. This is not, however, to be taken to exclude the applicability of the invention to use, if desired, in connection with bottles having corks or other similar closures that serve purely as a plug and which do not have an external skirt.
Nevertheless, problems have arisen in connection with certain kinds of cap in that fracture is found to be present occasionally in the neck or finish of the glass container after a cap has been applied. This fracture takes the form of cracking, often giving rise to only a very small crack but sometimes sufficient to cause a piece of glass actually to separate from the remainder of the container. This piece of glass will usually be held in place by the cap but will fall off, either externally or into the container itself, when the container is eventually opened. Fractures are found to occur during the actual capping process, and a problem arises in that, because they are generally in a part of the container hidden by the cap, inspection by known methods relying on the use of light, such as are mentioned above, is either difficult or impossible.
The use of acoustic waves emitted by various materials--commonly referred to as acoustic emission as an aid to detecting cracks in certain materials is known per se. It has for example been proposed for such purposes in connection with the bonding of electronic components, or in inspecting welds in metal; riveted, brazed or soldered joints in metal; and indeed in connection with any metal-working application in which areas of high stress, liable to give rise to cracking of the metal, are set up.
The stress waves which constitute the so-called "acoustic" emission travel outwardly from their source in the form of spherical waves and are generated whenever the material concerned is under stress, and not only when it cracks or otherwise fractures. However, in the event of fracture, energy is of course dissipated at a higher rate than if the stresses occurring are such as not to cause fracture. There are commercially-available acoustic emission crack detector systems which are used for the detection of fractures in the materials mentioned above. These systems can measure the magnitude of acoustic emission in terms of various parameters; as is the case in the systems disclosed in GB-A-1416082 and discussed above, the parameters for any particular practical application are chosen, according to the requirements of the user, to suit best the particular material and stress levels concerned, and according to the kind of information required.
Many of the materials referred to above are not "brittle" in the sense in which the term is used herein; the systems referred to are concerned with individual inspection of components rather than with the automatic monitoring of products being produced repetitively at high speed in a production line.